This invention relates generally to the reduction of compressor noise. One possible application of the system is for gas turbine engines, and in particular, auxiliary power units.
To increase engine operational ranges and to prevent engine surge, gas turbine engines utilize bleed holes/slots, which bleed air off the engine gas flow path. Gas turbine engine compressors rotate at high speeds, and in some designs the gas flow becomes supersonic relative to some portion of the impeller blade. One result of this rotation is a series of shock waves generated at the blade passing frequency (BPF), where the BPF is a “pure tone” frequency at which compressor blades pass a given fixed point in space, which exceeds the broadband noise portion of the acoustic spectrum. As pressure waves propagate from the near field at the compressor blade tip into the far field inside the inlet duct, they degenerate into a multi-tone sound spectrum characterized as “buzz saw” noise.
In addition to buzz saw noise generation, instances of supersonic flow in the region of the compressor blade tip causes pressure spikes to occur due to pressure perturbations/discontinuities across the pressure and suction sides of the compressor blades. This phenomenon results in the generation of pressure waves at a harmonic of the BPF frequency. These pressure waves can interact with and exit through the bleed holes/slots and result in the generation of significant amounts of sound power being generated by the compressor.